The present invention relates to an AGC system for a wide band tuner, particularly to the improvement on noise figure (NF) of a wide band TV tuner suitable for forming a double superheterodyne system.
In recent years many proposals for a wide band TV tuner employing a double superheterodyne system have been made. This is because, in the double superheterodyne system, an image frequency interference or the like influenced on a TV picture may be side-stepped or suppressed by means of an appropriate selection of higher intermediate frequency. Further, it is possible to receive not only TV signals of VHF and UHF frequency bands but cable TV signals etc., with the expansion of receiving frequency range of the tuner, and also possible to simplify the tuner configuration to reduce the manufacturing cost.
A typical wide band TV tuner will be explained below with referring to FIG. 1. In this figure numeral 1 denotes an input terminal for receiving an input signal from an antenna (not shown). The antenna input signal is supplied to an automatic gain controlled circuit (hereinafter referred to as the "AGC circuit") 2, and the level of magnitude of input signal is properly controlled therein. When the input signal or receiving signal is a VHF signal (inclusive of a cable TV signal), an output signal of the AGC circuit 2 is applied to a wide range radio frequency amplifier for VHF band (a VHF RF amplifier) 5a, through a VHF band-pass filter (BPF) 3a and 3b and a frequency band selector 4. The above output signal of AGC circuit 2 passes either the BPF 3a or 3b according to its specific frequency. An output signal of the selector 4 is amplified by the RF amplifier 5a. An output signal of the RF amplifier 5a is supplied to a UHF/VHF selection switch 7 through a frequency selective circuit of a VHF low-pass filter (LPF) 6a. The LPF 6a sufficiently suppresses the frequency components of image interferences. While the said receiving signal from antenna is a UHF signal, the output signal of the AGC circuit 2 is applied to a wide range radio frequency amplifier for UHF band (a UHF RF amplifier) 5b through a BPF 3c which passes frequency components of UHF. An output signal of the RF amplifier 5b amplified therein is supplied to the UHF/VHF selection switch 7 through a frequency selective circuit or a UHF LPF 6b. The components 3 to 6 form an RF circuit 40.
The signals supplied to the UHF/VHF switch 7 is alternatively selected by the switch 7 as a selected signal corresponding to a VHF signal or a UHF signal. The selected signal is applied to a first frequency mixer 8. Also applied to the mixer 8 is a frequency conversion signal obtained from a local oscillator 9. The mixer 8 converts the VHF or UFH signal inputted therein to a first IF signal by mixing the input signal with the frequency conversion signal.
The first IF signal filtered by a first IF BPF 31 is amplified by a first IF amplifier 30 and then filtered again by a first IF BPF 32. The components 8, 9, 30, 31 and 32 form a first superheterodyne section. An output signal of this section or a filtered output of the BPF 32 is supplied to a second superheterodyne section 34 which may have a well-known single superheterodyne configuration. In the section 34 the output of BPF 32 is converted into a second IF signal available for a conventional single superheterodyne tuner. The section 34 outputs a demodulated picture signal and generates a control voltage V.sub.AGC for AGC which corresponds to the magnitude of the aforementioned antenna input. The control voltage V.sub.AGC is applied to the AGC circuit 2 for gain-controlling.
FIG. 2 shows a conventional circuit of UHF/VHF selection switch 7. In this figure numerals 6a and 6b indicate VHF LPF and UHF LPF which are prestages of the switch 7. The switch 7 includes a switching diode 11 whose anode is connected to the LPF 6a, a switching diode 12 whose anode is connected to the LPF 6b, a DC blocking capacitor 13 for coupling the cathodes of the diodes 11 and 12 with the first frequency mixer 8 of following stages, and a resistor 14 connected between the output nodes of the diodes 11, 12 and the circuit ground. The cathodes or output nodes of diode 11 and 12 is refered as a coupling node N.
The UHF/VHF selection switch 7 operates as follows. Suppose that a VHF receiving signal as mentioned before is fed to the switch 7 through the wide band RF amplifier 5a and the LPF 6a. In this case, when a power supply voltage of VHF circuit is applied to the switching diode 11 in a manner that the power supply voltage forwardly biases the diode 11, a DC current is flows through the diode 11 and the resistor 14 to a circuit ground thereby to render the switching diode 11 conductive. Then, the VHF signal is fed through the DC blocking capacitor 13 to the first frequency mixer 8. At this time, since the voltage potential of a UHF power supply is zero, the switching diode 12 is reversely biased to render the diode 12 nonconductive.
When a UHF receiving signal is fed through the wide band RF amplifier 5b and the LPF 6b to the switch 7, the switching diode 12 of UHF side is rendered conductive and the switching diode 11 of VHF side is rendered nonconductive, contrary to the aforementioned case. In this case only the UHF signal is fed to the first frequency mixer 8 through the DC blocking capacitor 13.
One of the differences between a double superheterodyne type tuner and a single superheterodyne type tuner appears in an AGC circuit. In case of a single superheterodyne tuner, the AGC is generally performed at a tuning amplifier or an RF amplifier. While, in case of a double superheterodyne tuner in which a wide band RF amplifier as mentioned is used, it is hard to realize an AGC at the RF amplifier. The difficulty of this is due to the wide frequency range requirement for covering VHF and UFH bands and a necessity of a forward AGC. That is, in a wide band RF amplifier with an AGC circuitry, its frequency response is liable to vary with the AGC operation. In other words, it is difficult to maintain the flat frequency response of RF amplifier within the full AGC operating range. Further, the use of a forward AGC is strongly desired because the harmonic distortion of forward AGC is smaller than that of reverse AGC. In a wide band RF amplifier for VHF and UHF, however, reducing a forward AGC to practice is almost impossible.
From reasons mentioned above, in a wide band RF amplifier, the special AGC circuit 2 as shown in FIG. 1 is used. When such AGC circuit 2 is provided in the prestage of RF amplifiers 5a and 5b, an input-referred noise figure (NF) of the tuner is deteriorated with the increase of degree of AGC. In this case an NF of the double superheterodyne system in a medium field strength is more deteriorative than that of the single superheterodyne system. Thus, the picture S/N of a TV employing the double superheterodyne system is degraded within a medium field strength area.